This invention relates to methods and means for containing molten metal such as aluminum, whether flowing or stationary, in such fashion as to keep the metal in molten state at a desired temperature. In a more specific sense, the invention is concerned with vessels for holding molten metal, e.g. non-ferrous metal such as aluminum, which is to be kept molten at a temperature of not more than about 1350.degree. F., in such fashion that significant economy of heat is achieved while the molten metal is effectively maintained at a selected temperature.
In a particularly important sense, the invention is related to pipeline for molten metal, notably molten aluminum, so arranged and operated that molten metal can be transported through the pipeline over relatively long distances with essentially no fall in temperature and no undue consumption of applied heat.
Heretofore, various efforts have been made to transport molten metal, i.e. molten at the temperature described above, as by troughs or pipes so that, for example, the metal may be conducted from a melting or holding furnace to a fabricating locality, such as a casting machine which is required to be supplied with such metal either intermittently or continuously. As commonly made with appropriate insulation, metal conducting troughs have often been employed, with some advantage in that they allow access for removal of so-called skulls after each use, but such troughs tend to dissipate heat so that there is considerable drop in temperature of the metal between entry and exit of the system, limiting the attainable length of travel and creating a risk of cooling the metal below a temperature appropriate for the selected casting operation.
Insulated pipes have been proposed, having a refractory lining in a strong steel shell, and while they have a further advantage in reducing the exposure of the molten metal to air, thus avoiding unwanted oxidation, they have nevertheless been characterized by difficulties similar to those of open troughs. Moreover, in all of these cases, there is some problem in starting up the system, in that the first flow of metal must lose some heat in order to raise the temperature of the trough or pipe, with special risk of casting machine difficulty at this particular time.
Some efforts have been made to design troughs with refractories of very high insulating property and also by adding covers which might include heating devices. It has nevertheless remained difficult to avoid temperature drops along the system and to control the operation so as to avoid overheating the metal, or waste of heat, or fall of metal temperature too close to the melting point. The energy loss in many heated lines is a necessary penalty of the system, and it adds to the operating cost. Indeed, under the best of previous circumstances, trough lengths have not usually exceeded about 10 meters, or perhaps with high metal flow rate, as much as a total of 20 meters.
In one form of previous system utilizing refractory, insulated troughs or launders to carry the metal from a central locality to equipment utilizing it, efforts have been made to keep them substantially horizontal so as to have the least air contact of the metal surface by turbulence, and at the same time electric heaters have been disposed over the passing metal, or the metal is held and heated in a local furnace, with the object of keeping the metal molten while trying to minimize oxidation.
In another system, the transfer is through holding pans, comprising an open pan-like shell of steel, lined with a thick layer of insulation. The outside of the steel shell has cooling pipes, chilled by passage of fluid coolant. This arrangement is said to avoid cracking or damage to the refractory lining when the system is heated up by the molten metal or cooled down when the metal is drained; yet it neither saves heat nor insures economical maintenance of desired metal temperature.
In most of these prior devices, there remains the problem of loss of heat as the molten metal progresses along the system, with corresponding limitation, as explained above, on the distances to which the metal can be transferred. At the same time, in such covered trough arrangements as had included some heat supply (as in the trough covers), operation has not been very economical and control of temperature may not be satisfactory; there may even be variations affecting the metallurgical uniformity of products cast from the metal.